A vaccine is a biological preparation which is used to establish or improve immunity to a particular disease. Vaccines can be prophylactic (e.g. to prevent or ameliorate the effects of a future infection by any natural or "wild" pathogen), or therapeutic (e.g. vaccines against cancer are also being investigated; see cancer vaccine).
An important challenge of immunization safety is to ensure vaccine quality and safety. This must occur from the development stages through clinical trials, vaccine production, quality control and distribution, up to the point of use, where adequate practices for immunization must be in place.
Inactivated (killed) vaccines cannot cause an infection, but they still can stimulate a protective immune response. Viruses are inactivated with chemicals such as formaldehyde.
Toxoid vaccines are made by treating toxins (or poisons) produced by germs with heat or chemicals, such as formalin, to destroy their ability to cause illness. Even though toxoids do not cause disease, they stimulate the body to produce protective immunity just like the germs' natural toxins.
Some vaccines are made by using only parts of the viruses or bacteria. These vaccines cannot cause disease, but they can stimulate the body to produce an immune response that protects against infection with the whole germ.
Disease causing organisms have at least two distinct effects on the body. The first effect is very obvious: we feel sick, exhibiting symptoms such as fever, nausea, vomiting, diarrhea, rash, and many others. Although the second effect is less obvious, it is this effect that generally leads to eventual recovery from the infection: the disease causing organism induces an immune response in the infected host. As the response increases in strength over time, the infectious agents are slowly reduced in number until symptoms disappear and recovery is complete.